I dare say an OS washer for a 10/15/20 will fit-you can get these as an individual spare, if a proper TT one proves elusive..... [dependinging in the measurements of the shaft and crankcase nose diameters] As an alternative-and requiring the same data-modelfixings.co.uk do a nice line in shim washers, in sizes from 0.1mm thickness up. Somewhere in their range you should find something suitable...and they're in your neck of the woods, so to speak....
ChrisM
'ffkiwi'